Sometimes when my teammates have fallen i cant revive them because to many mobs scramble the place, even when i use grapple or a stun i cannt seem to hit my ally.

maybe:
add an option to press a icon on your party list or screen wich will try to auto revive that teammate if you stand close enhough

or you can collect the soul of your fallen ally after clearing all the enemy arround his body and then a side quest will appear and the remaining players must
(1)clear x amount of monsters wich increases by the number of times people have died in the party in order to give the first fallen ally a respawn token (a Orb need to be filled)
(2) search the map for respawn token
(3) search the map for a mini boss wich will reward after killed, a respawn for first fallen ally
(4) search for an area with a cage (with fallen party member in it), clear the extra sturdy enemies in order to release your ally